I presume it's for the same reason we don't preserve homebrew games; it would be a nightmare for us to catalog. For every Tom, Dick, and Harry who makes his own poor quality NES game, puts it on a cartridge, and sells it to somebody, we need to buy the game, dump it, and DAT it. What if I only sell a couple copies of the game to my friends; does it still need to be preserved by us? What if it uses some obscure, custom mapper that will never be emulated?

We need to place a limit on what we want to preserve, and we've set that limit to games made during the console's lifetime that are not intentional pirates. That's not to say that all pirates, hacks, homebrews, and new games are useless from a preservation standpoint, but we don't preserve them.

Super Turrican Director's Cut isn't really a new aftermarket game. It is more accurate to say that it is a prototype of the original Super Turrican game, which happens to have more content. I would definitely say it belongs on the No Intro list, along with Star Fox 2.